This invention relates in general to bi-directional backstopping clutches and in particular to an improved hardened insert and brake shoe adapted for use in such clutches.
Bi-directional motors are well known devices which are adapted to selectively rotate an output shaft in either a clockwise or counterclockwise direction. Usually, the output shaft is connected to a driven device for rotation therewith, and rotation of the output shaft transmits torque to the driven device. In certain situations, however, the driven device may tend to back drive torque to the motor. In order to prevent this back driving action, a backstopping clutch is usually coupled between the output shaft of the motor and the input shaft of the driven device. A typical bi-directional backstopping clutch includes an input yoke connected to a pair of opposed brake shoes disposed within a hollow cylindrical brake drum. The brake shoes are generally semi-circular in shape, having arcuate braking surfaces which correspond to the inner surface of the brake drum. A pair of drive pins are disposed between the brake shoes, normally in parallel alignment. The drive pins extend into apertures formed in an output member.
Under normal operating conditions, the input yoke rotates the brake shoes, the drive pins, and the output shaft within the brake drum, thereby transmitting torque through to the driven device. However, if the driven device attempts to back drive torque through the clutch, the output member causes the drive pins to become skewed. As a result, the brake shoes are spread apart from one another into locking frictional engagement with the inner surface of the brake drum. In this manner, the clutch prevents back driving of the motor by the driven device.
The brake shoes are typically provided with hardened wear resistant inserts. The inserts are disposed in respective slots formed in the arcuate outer braking surfaces of the brake shoes. When the brake shoes are moved apart from one another during braking as described above, the inserts frictionally engage the inner surface of the brake drum. The inserts are provided to reduce wear and to reduce repair costs when the braking surfaces have become worn. In the past, these inserts have been formed from solid tungsten carbide
Although inserts formed of this material function satisfactorily, they are quite difficult to manufacture. The extreme hardness of the tungsten carbide material makes them very difficult to grind to proper dimensions. Furthermore, because of the nature of the material itself and the difficulties involved in machining such material, tungsten carbide inserts are quite expensive. At the present time, a single insert manufactured from solid tungsten carbide may cost up to twenty dollars. Accordingly, it would be desirable to provide an insert which is formed from a less expensive material and which is easier to machine than tungsten carbide, yet which functions satisfactorily in bi-directional backstopping clutches. Alternatively, it would be desirable to provide a backstopping clutch which does not require the use of any inserts whatsoever.